This invention relates to a pressure control apparatus, and in particular to an improved arrangement for a control apparatus of pressure equipment, such as braking apparatuses and door opening and closing apparatuses, driven by a motor in a vehicle.
In conventional braking apparatuses for vehicles, for example, only a foot braking force was available and so the braking effect was not satisfactory. However, in recent braking apparatuses a greater braking effect has been required due to road conditions and other factors, and there have been an increasing number of vehicles that utilize a braking force magnification apparatus which produces a greater braking effect than can be achieved by the foot alone. A braking force magnification apparatus with pressure generating means included therein can generate a pressure different from atmospheric pressure so that the difference between the generated pressure and atmospheric pressure may be available for braking operations. Accordingly, a braking force magnification apparatus requires a pressure control apparatus which can sustain the generated pressure at a predetermined value.
One example of such a pressure control apparatus used in the past is shown in FIG. 1 by a circuit diagram. In the figure, the pressure control apparatus is generally shown by a reference numeral 10 in which a vacuum pump or compressor 12 and a DC motor 14 connected to the pump 12 form a pressure generating means. The vacuum pump or compressor 12 will be considered to be a pump for the remainder of this explanation. The DC motor 14 is connected to the positive terminal of a DC power source 16 through a key switch 18, and the negative terminal of the source 16 is grounded. The motor 14 is also connected to a control circuit 20 surrounded by dotted lines. The control circuit 20 is connected to ground via sensor 22, and includes resistors R1 and R2 connected in series between one terminal of the key switch 18 and ground. One junction of the resistors R1 and R2 is connected to one of the contacts of the sensor 22, the other contact being grounded, and also to the base of a driver transistor Q1 whose emitter is grounded and whose collector is connected through a resistor R3 to the key switch 18 and to the base of a power transistor Q2, driven by the transistor Q1, whose emitter is grounded and whose collector is connected to the DC motor 14 as well as to the anode of a diode D which forms a parallel combination with the DC motor 14 connected to the key switch 18. Another sensor 24 is serially connected to an alarm lamp 26 between the series combination of the power source 16 and the key switch 18, as shown in the figure.
The vacuum pump 12 produces a negative gauge pressure in a vacuum booster or a reserve tank (both not shown), the negative pressure serving as a mechanical power source for braking operations. The sensors 24 and 22 are provided within the vacuum booster or the reserve tank such that they sense first (lower) and second (higher) predetermined threshold values of the negative gauge pressure respectively, at which time they are actuated.
In FIG. 1, the pressure generating means includes the DC motor 14 and the vacuum pump 12 while it may include the DC motor 14 as well as a compressor which produces a positive gauge pressure also serving as a mechanical power source for braking operations. In this case, the sensors 22 and 24 respond to first (lower) and second (higher) threshold values of the positive gauge pressure. For the sake of convenience, the following description will be made only with reference to a vacuum pump.
In operation, a conventional apparatus thus constructed closes the contacts of the sensors 22 and 24 when the absolute gauge pressure within the vacuum booster or reserve tank is approximately equal to atmospheric pressure before the key switch 18 is closed to make negative gauge pressure. In this state, when the key switch 18 is closed, the alarm lamp 26 is lighted through the closed contacts of the sensor 24 while the driver transistor Q1 is not conductive because its base is grounded through the closed contacts of the sensor 22, and so the power transistor Q2 is made conductive by the base current supplied through the resistor R3 from the power source 16. Therefore, the DC motor 14 is energized to decrease the absolute pressure, i.e. to direct negative the pressure within the vacuum booster through the vacuum pump 12, and then to increase the negative gauge pressure.
FIG. 2 will now also be referred to regarding the relationship between the operations of the sensors 22 and 24, and the degree of vacuum. As schematically shown in the figure, when the vacuum booster is negatively pressurized to the first predetermined threshold value A along the direction of the arrow, the sensor 24 is firstly actuated or made open and maintains this state as far as the gauge pressure goes more negative. In this state, the alarm lamp 26 is extinguished.
When the negative gauge pressure further rises to the second predetermined threshold value B, the sensor 22 is now actuated or made open and maintains this stage as far as the gauge pressure goes more negative. Therefore, the driver transistor Q1 is made conductive by its base current flowing through the closed key switch 18 and the resistor R1 from the power source 16 to ground, thereby making the power transistor Q2 non-conductive. Consequently, the DC motor 14 is deenergized to stop the pressurizing operation in the negative direction.
After the operation of the DC motor 14 is stopped, the negative gauge pressure within the vacuum pump now begins to fall gradually towards the second predetermined threshold value B, at which the contacts of the sensor 22 are closed and therefore the transistor Q1 is made non-conductive and the transistor Q2 is made conductive to energize the DC motor 14 as described above. The repetition of these operations will cause the vacuum booster or reserve tank to be maintained at the second predetermined negative gauge pressure B.
If the DC motor 14 or the vacuum pump 12 malfunctions for some reason, then the negative gauge pressure continues to fall towards the first predetermined threshold value A, at which the contacts of the sensor 24 are closed as shown in FIG. 2 to light the alarm lamp 26, signalling a warning.
Thus it is disadvantageous that the pressure control apparatus in the prior art employs two sensors, since the increased number of the components raises costs.
Accordingly, it is an object of the invention to provide a pressure control apparatus which has a simple arrangement and low cost.